I like both the Atticus and Auteur as a compliment to my HD800. Atticus is more of a classic contrast like the LCD-2 - more bassy and fun with a romantic midrange - I especially like it with OTL tube amps where it takes on an almost horn speaker-like character - almost warm and intimate to a fault, but pulls up just short of that in a spot where you get ALL THE FEELS!!! IMHO, YMMV. Preference for it is highly subjective, but it’s a very sweet, smooth, relaxing headphone.
Auteur is a different beast altogether, being a lot more neutral/balanced, but ultra punchy with an elite combo of clarity and presence in the low frequencies, and an incredibly sweet midrange. Definitely a lot more resolving and offers more clarity across the board, whereas the Atticus puts more of a spotlight on the lead instrument. Sort of takes some elements from the LCD-X and LCD-4.
Some people might feel that a more neutral balance might be redundant with the HD800/S/SDR and go with a stronger contrast like Atticus or LCD-2... others might prefer hearing a headphone with a slightly different presentation that’s still in the ballpark of neutral like Auteur or LCD-X... personally, I like having both!